The presence of sites of uptake and degradation of 5-HT in the craniofacial region of the mouse embryo, together with the ability of uptake inhibitors to cause craniofacial malformations in whole embryo culture have implicated serotonin (5-HT) as a morphogen during craniofacial development. However, much of this evidence is circumstantial. Therefore, it was the goal of this progect to investigate underlying mechanisms by which serotonin might be able to modulate craniofacial development. The possibility that cranial neural crest migration was involved was addressed by developing a modified Boyden chamber assay to investigate serotonergic regulation of this process. Migration of cells from explants of cranial neural crest, from E9 mouse embryos (plug day = E1), were found to be stimulated in a dose-dependent manner by 5-HT. It was also been demostrated that this effect is mediated by the 5-HTIA receptor. Two in vitro models were developed to investigate the effects of 5-HT on mesenchymal cell differentiation. Micromass cultures from mandibular mesenchyme of E12 mouse embryos were cultured in serum containing medium, and organotypic mandibular explant cultures from E13 mouse embryos were cultured in fully defined medium. The serotonergic regulation of expression of cartilage proteoglycan core protein, a marker of chondrogenesis, tooth germ development and two morphregulatory molecules (S-IOO,B and tenascin), which might indirectly mediate effects of 5-HT, were investigated. It appears that 5-HT does indeed regulate expression of these morphoregulatory molecules and modulate migration, differentiation and morphogenesis within the craniofacial structures. However, rather than indirectly affecting these processes by modulation of morphoregulatory molecules, such as S-100, and tenascin, it appears that a direct mechanism is responsible for these effects. The possibility remains, however, that other molecules might act as intermediaries in the serotonergic regulation of craniofacial development.